The present invention relates to medical devices and, more particularly, to medical devices employed for indicating a position of a device located within a body and being imaged on an X-ray imaging system.
A stereotaxic catheter is employed, for example, to penetrate the brain of a human being for local administration of an anti-cancer drug or cancer destroying radiation. Such a use is described in allowed U.S. patent application Ser. No. 779,285, now matured into U.S. Pat. No. 4,681,122, wherein a multi-lumen catheter includes a microwave antenna in one of the lumens for elevating the temperature of tissue surrounding the antenna. A temperature sensor in a second of the lumens senses the tissue temperature to permit temperature control with a precision effective to damage cancerous tissue without excessive damage to healthy tissue.
The catheter, and particularly the microwave antenna and temperature sensor within the catheter, must be positioned precisely within the tissue in order to localize the hyperthermic effect of the microwave radiation. Precise angular positioning of a catheter is aided by a positioning device such as a stereotaxic array plug disclosed in U.S. Pat. No. 4,629,451. In this device, a plug is threaded into an opening in a patient's skull. The plug, supports guide holes for guiding the insertion into the brain of a catheter at precise angles. A radiopaque marking pattern on the outer surface of the plug provides a calibration pattern on an X-ray image.
Longitudinal positioning of the catheter in the brain is preferably aided by an X-ray image, whereby a practitioner may see the relationship between the catheter and the portion of the brain requiring treatment. The catheter itself is generally made of a resin such as, for example, a silicone plastic, which does not present substantial X-ray absorption. Thus, means are required to enhance the visibility of the catheter on an X-ray image.
One technique for enhancing the visibility of the catheter, disclosed in the referenced allowed U.S. patent application, includes placing one or more radiopaque marking bands about the surface of the catheter. In a preferred embodiment, a pattern of equally spaced bands about the catheter are spaced a predetermined distance apart along the catheter axis. The spacing, and the known relationship between the locations of the bands and the microwave antenna, provides a practitioner viewing an X-ray image displaying the bands with the information required to permit precise longitudinal positioning of the catheter.
The bands are conventionally formed using a liquid containing a non-toxic material having a large X-ray signature. The liquid is applied in a series of stripes, about the circumference of the catheter and then dried. One type of radiopaque marking material found satisfactory from the standpoint of its X-ray signature is a gold powder mixed with a silicone adhesive to form a paste. Other compositions having the required properties of X-ray signature and non-toxicity may be employed. One such composition is Tantalum Powder-Type 268/1905 ZM-414 (-325 Mesh size) distributed by Fansteel Metals.
Silicone rubber is selected as the material for catheters at least partly because such material rejects the attachment of contaminants to its surface. Although desirable from the standpoint of therapy, such a property makes the surface of a catheter a poor host for radiopaque marking bands. Thus, during insertion of a catheter into brain tissue, and its later removal therefrom, a substantial likelihood exists that some of the marking material may rub off and remain in the tissue. It is, of course, undesirable to permit the deposit of a foreign material within brain tissue. In addition, if a substantial part of the marking bands are removed during insertion, their visibility on an X-ray image may be degraded.
One possibility may be to form annular grooves in the surface of the catheter within which the marking material is deposited. The mechanical keying between the marking material and the catheter improves the adhesion of the marking material. Besides adding to the cost of producing a catheter, however, such grooves may produce a rougher surface which could increase tissue damage during insertion and removal.